This study is to determine the efficacy of momelotinib (MMB) versus ruxolitinib in participants with primary myelofibrosis (PMF) or post-polycythemia vera or post-essential thrombocythemia myelofibrosis (post-PV/ET MF) who have not yet received treatment with a Janus kinase inhibitor (JAK inhibitor).

Participants will be randomized to receive either MMB or ruxolitinib for 24 weeks during a double-blind treatment phase, after which they will be eligible to receive open-label MMB for up to an additional 168 weeks. After discontinuation of study medication, assessments will continue for 12 additional weeks, after which participants will be contacted for survival follow-up approximately every 6 months for up to 5 years from the date of enrollment.

Total symptom score (TSS) is defined as the proportion of participants who achieve a ≥ 50% reduction in TSS from baseline to Week 24 as measured by the modified Myeloproliferative Neoplasm Symptom Assessment Form Total Symptom Score (MPNSAF TSS) v2.0 diary.

RBC transfusion independence is the proportion of participants who are transfusion independent at Week 24, defined as absence of RBC transfusions and no hemoglobin level below 8 g/dL in the prior 12 weeks.

RBC transfusion dependence is the proportion of participants who are transfusion dependent at Week 24, defined as at least 4 units of RBC transfusions, or a hemoglobin level below 8 g/dL in the prior 8 weeks.

Classified as high risk OR intermediate-2 risk as defined by the International Prognostic Scoring System (IPSS) for PMF, or intermediate-1 risk (IPSS) associated with symptomatic splenomegaly, hepatomegaly, anemia (hemoglobin < 10.0 g/dL), and/or unresponsive to available therapy

Acceptable laboratory assessment obtained within 14 days prior to the first dose of study drug:

Absolute neutrophil count (ANC) ≥ 0.75 x 10^9/L in the absence of growth factor in the prior 7 days

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Please refer to this study by its ClinicalTrials.gov identifier: NCT01969838